1. Field of the Invention
The present invention relates to a luminescent display device for analog display and, more particularly, to a luminescent analog-display device for displaying numerical information in an analog fashion, for instance, in the form of a bar graph or the like.
2. Description of the Prior Art
As one of the types of the analog display devices for indicating or displaying numerical information in the form of, for instance, a bar graph, there have been luminescent analog-display devices of the type using luminescent material which emits light when bombarded with thermions.
Such a luminescent analog display device is composed of a substrate of insulating material, a plurality of segment anodes provided on the substrate and arranged, for instance, rectilinearly and each coated with a luminescent material layer, a mesh-like control electrode, or control grid, provided above and in the vicinity of the segment anodes, and a filamentary cathode, or filament, provided above the control grid and adapted to emit thermions when heated, the segment anodes together with the control grid and filament forming a display section, in which an anode voltage is given selectively to the segment anodes so that the number of segment anodes to which the anode voltage is given corresponds to numerical information to be displayed, and thereby the segment anodes to which the anode voltage is selectively given emit light to display the numerical information in an analog fashion, for instance, in the form of a bar graph.
The above-mentioned luminescent analog-display device has advantages in that it can emit light of high-quality color and therefore can be used in a dark environment and in that it has no moving part and therefore is free of malfunctions due to mechanical vibrations or the like and high in response speed.
In order to drive the above-mentioned luminescent analog display device, the following driving systems are usually adopted:
In an example of the driving systems, a plurality of segment anodes each coated with a luminescent material layer are electrically connected independently of one another to external terminals through which an anode voltage is selectively given to the anode segments so that the amount of anode voltage corresponds to numerical information to be displayed, and thereby analog display is carried out in a static fashion.
According to the above-mentioned driving system, it is possible to make the segment anodes emit light by a relatively low anode voltage. However, it has disadvantages in that, as the number of segment anodes is increased for the purpose of increasing the quantity of display or improving the reading accuracy, the number of external terminals increases and also the number of driving circuits increases which give the segment anodes a driving voltage.
In another example of the driving systems, a number of segment anodes rectilinearly arranged and each coated with a luminescent material layer are divided into a plurality of groups each having a predetermined number of segment anodes so that the segment anodes arranged at the corresponding positions in the above groups are electrically connected in common and also are connected to external terminals respectively, and a plurality of control grids are provided independently of one another for the respective groups, each being connected to a separate external terminal. In this example, the above-mentioned control grids are scanned in regular succession and in a time-sharing manner, and a driving voltage corresponding to numerical information is given to the segment anodes in synchronization with the scanning signal for the control grid thereby to perform analog display.
According to the above-mentioned driving system, the number of the external terminals corresponds to the sum of the number of the segment anodes of each group and the number of the control grids provided for the respective groups. Therefore, even when the number of segment anodes of each group or the number of groups is increased, the increase of the number of external terminals can be minimized. Accordingly, fine and accurate display can be performed using a small number of external terminals. In this example, however, the increase of the number of groups will result in the decrease of the so-called duty factor, i.e., the ratio of the duration of light emission of each segment anode to the scanning period of the control grid. Therefore, the driving voltage given to the segment anode must be kept high to obtain the desired luminance.